49
2.
Excessive imbalance in the capillary bridge.
The ability of the bridge to cancel pump pulsations is directly proportional to the degree
of balance. Normal balance is better than 1% as calculated in Section C.3.a and is
unacceptable above 3%. The higher the imbalance, the lower the pump pulsation will need
to be.
3.
Excessive pump pulsation, as measured by the IP signal/noise in Section C.3.c.
If this is excessive (IP
S/N
< 200), check the operation of the pneumatic pulse dampener.
Drain the tube with the flow off, recap, and measure IP noise again. Finally check the
operation of the pump itself.
D.1.b Random (White) Noise in the DP Baseline
Random noise in the DP transducer is usually electronic rather than flow related. To
confirm that this is the case, simply turn off the pump flow. If the noise continues, it is electronic
in nature. Normal random (white) noise as measured in section C.3.b should be 0.3 mV or less.
If this level is exceeded with the flow off, check the DP transducer. To check this as a possible
source of noise, remove the cable from the DP transducer. If the noise disappears, the
transducer is at fault and will need to be replaced. If the noise does not disappear, electronic
noise elsewhere in the output board circuit is indicated.
D.1.c Sporadic Noise in the DP Baseline
Sporadic noise in the DP baseline, such as a sudden decrease in the baseline, or a
spike, etc., is usually caused by flow problems such as a faulty pump seal, check valve, or bad
column frit, or occasionally by the passage of an air bubble through the system. This can be
confirmed by observing the IP baseline overlaid together with the DP baseline. If the IP
baseline shows a large change coincident with the change in the DP baseline, some fault in the
flow stability is indicated.
D.2 PROBLEMS IN THE LIGHT SCATTERING BASELINE
D.2.a General Instrument Function
If there is any question as to the instrument’s function, or if the instrument has been
moved around, the response of the instrument should be compared to the results of a known
standard, as described in Section C.3.e.
D.2.b Baseline Noise
If there is any question about the source of baseline noise, the detector should be taken
off-line and pure clean solvent (methanol or THF works well) should be put in the cell of the light
scattering unit with a syringe. Once the fresh solvent has thoroughly flushed out the system, the
baseline noise should be less than 2 mV peak to peak on average. If the instrument shows